Displacement measuring interferometers monitor changes in the position of a measurement object (e.g., a plane mirror or retroreflector) relative to a reference object based on an optical interference signal. The interferometer generates the optical interference signal by overlapping and interfering a measurement beam reflected from the measurement object with a reference beam reflected from the reference object.
In many applications, the measurement and reference beams have orthogonal polarizations and different frequencies. The different frequencies can be produced, for example, by laser Zeeman splitting, by acousto-optical modulation, or internal to the laser using birefringent elements or the like. The orthogonal polarizations allow a polarizing beam splitter to direct the measurement and reference beams to the measurement and reference objects, respectively, and combine the reflected measurement and reference beams to form overlapping exit measurement and reference beams. The overlapping exit beams form an output beam that subsequently passes through a polarizer.
The polarizer mixes polarizations of the exit measurement and reference beams to form a mixed beam. Components of the exit measurement and reference beams in the mixed beam interfere with one another so that the intensity of the mixed beam varies with the relative phase of the exit measurement and reference beams. A detector measures the time-dependent intensity of the mixed beam and generates an electrical interference signal proportional to that intensity. Because the measurement and reference beams have different frequencies, the electrical interference signal includes a “heterodyne” signal having a beat frequency equal to the difference between the frequencies of the exit measurement and reference beams. If the lengths of the measurement and reference paths vary relative to one another (e.g., by translating a stage that includes the measurement object), the measured beat frequency includes a Doppler shift equal to 2νnp/λ, where ν is the relative speed of the measurement and reference objects, λ is the wavelength of the measurement and reference beams, n is the refractive index of the medium through which the light beams travel (e.g., air or vacuum), and p is the number of passes to the reference and measurement objects. Changes in the relative position of the measurement object correspond to changes in the phase of the measured interference signal, with a 2π phase change substantially equal to a distance change L of λ(np), where L is a round-trip distance change (e.g., the change in distance to and from a stage that includes the measurement object).
Unfortunately, this equality is not always exact. In addition, the amplitude of the measured interference signal may be variable. A variable amplitude may subsequently reduce the accuracy of measured phase changes. Many interferometers include non-linearities such as so-called “cyclic errors.” The cyclic errors can be expressed as contributions to the phase and/or the intensity of the measured interference signal that have a sinusoidal dependence on the change in optical path length pnL. In particular, the first harmonic cyclic error in phase has a sinusoidal dependence on (2πpnL)/λ and the second harmonic cyclic error in phase has a sinusoidal dependence on 2(2πpnL)/λ. Higher harmonic cyclic errors can also be present.
Another source of errors is related to environmental effects such as gas turbulence. Interferometric displacement measurements in a gas are subject to environmental uncertainties, particularly to changes in air pressure and temperature; to uncertainties in gas composition such as resulting from changes in humidity; and to the effects of turbulence in the gas. Such factors alter the wavelength of the light used to measure the displacement. Under normal conditions the refractive index of air, for example, is approximately 1.0003 with a variation of the order of 1×10−5 to 1×10−4. In many applications, the refractive index of air should be known with a relative precision of less than 0.1 ppm (parts per million) to less than 0.001 ppm, these two relative precisions corresponding to a displacement measurement accuracy of 100 nm and less than 1 nm, respectively, for a one meter interferometric displacement measurement. For discussion of environmental uncertainties, see, for example, the article entitled “Residual Errors In Laser Interferometry From Air Turbulence And Nonlinearity,” by N. Bobroff, Appl. Opt. 26(13), 2676-2682 (1987), and the article entitled “Recent Advances In Displacement Measuring Interferometry,” also by N. Bobroff, Measurement Science & Tech. 4(9), 907-926 (1993).
One way to detect refractive index fluctuations is to measure changes in pressure and temperature along a measurement path and calculate the effect on the optical path length of the measurement path. Examples of mathematical equations for effecting such a calculation are disclosed in an article entitled “The Refractivity Of Air,” by F. E. Jones, J. Res. NBS 86(1), 27-32 (1981). An implementation of the technique is described in an article entitled “High-Accuracy Displacement Interferometry In Air,” by W. T. Estler, Appl. Opt. 24(6), 808-815 (1985).
Another, more direct way to detect the effects of a fluctuating refractive index over a measurement path is by multiple-wavelength distance measurement. The basic principle may be understood as follows. Interferometers and laser radar measure the optical path length between a reference and an object, most often in open air. The optical path length is the integrated product of the refractive index and the physical path traversed by a measurement beam. In that the refractive index varies with wavelength, but the physical path is independent of wavelength, it is generally possible to determine the physical path length from the optical path length, particularly the contributions of fluctuations in refractive index, provided that the instrument employs at least two wavelengths. The variation of refractive index with wavelength is known in the art as dispersion and this technique is often referred to as the dispersion technique.
An example of a two wavelength distance measurement system is described in an article by Y. Zhu, H. Matsumoto, T. O'ishi, SPIE 1319, Optics in Complex Systems, 538-539 (1990), entitled “Long-Arm Two-Color Interferometer For Measuring The Change Of Air Refractive Index.” The system of Zhu et al. employs a 1064 nm wavelength YAG laser and a 632 nm HeNe laser together with quadrature phase detection. A similar instrument is described in Japanese in an earlier article by Zhu et al. entitled “Measurement Of Atmospheric Phase And Intensity Turbulence For Long-Path Distance Interferometer,” Proc. 3rd Meeting On Lightwave Sensing Technology, Appl. Phys. Soc. of Japan, 39 (1989).
Interferometers utilizing the dispersion technique are can be included as components of scanner systems and stepper systems used in lithography to produce integrated circuits on semiconductor wafers. Such lithography systems typically include a translatable stage to support and fix the wafer, focusing optics used to direct a radiation beam onto the wafer, a scanner or stepper system for translating the stage relative to the exposure beam, and one or more interferometers. Each interferometer directs a measurement beam to, and receives a reflected measurement beam from, a plane mirror attached to the stage. Each interferometer interferes its reflected measurement beams with a corresponding reference beam, and collectively the interferometers accurately measure changes in the position of the stage relative to the radiation beam. The interferometers enable the lithography system to precisely control which regions of the wafer are exposed to the radiation beam.